Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor and control the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the linewidth (Critical Dimension) and sidewall angle (SWA) of features formed in or on it. There are various techniques for making measurements of the microscopic structures (features) formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the properties of the beam (intensity, polarization state) as a function of wavelength of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the properties of the scattered radiation as a function of angle of the reflected beam.
The target may be specially formed for the purposes of the monitoring and control measurements. Alternatively the target may be present already in product features being formed on the substrate. The target may be formed of resist material that has been patterned by the lithographic process, prior to etching of the substrate material. The resist material may or may not have been developed. Irrespective of these details of implementation, we can say the measurements seek to measure one or more parameters that characterize the target, such as parameters that describe the profile or shape of the target. For control of a lithographic process, a change in target profile may be used to control the lithographic apparatus (scanner). The change in target profile is from a reference process to a perturbed process. The reference process may be for example the process at a certain time on a certain apparatus, the process of an individual apparatus with the best yield, or the process in which an OPC (Optical Proximity Correction) model was created. The perturbed process may be a process exposed at a different time or on a different lithographic apparatus or coating/developing apparatus (referred to in the art as the “track”). The perturbed process may be implemented side-by-side with the reference process by introducing targets with biased parameters. Both processes may in fact be biased relative to an ideal process, so that the terms “reference” and “perturbed” are merely labels and are interchangeable for the purposes of the measurement itself. In known approaches to calculate the change in target profile, the absolute target profile is calculated for the reference process. Next the absolute target profile is calculated for the perturbed process. The change in target profile is then obtained by subtraction of these two absolute profiles. Such a method is a biased predictor of the change because one has to use prior information for the reconstruction of the profile based on observations made by scatterometry. Any metrology method that is biased has inherent problems in the confidence of the measurement. In addition, the creation of a recipe to measure the absolute target profile is a tedious job, requiring a skilled and experienced engineer.